Rapid diagnosis of La Crosse (LAC) arboviral encephalitis will be examined using techniques to detect viral antigen and early-appearing antibodies. LAC viral antigen has been detected in cryostat sections of skin biopsies obtained from patients with suspected LAC encephalitis. Specific fluorescence was detected only in about 1/3 of patients with greater then or equal to 1:80 acute phase HAI titers and never in patients with lower titers. Studies will be conducted using anticomplementary immunofluorescence techniques to determine if antigen-antibody complexes are the source of the fluorescence. F(AB)2 immunoglobulin fragments have been prepared and conjugated with fluorescein isothiocyanate in order to examine peripheral blood CSF and skin leukocytes for the presence of LAC viral antigen. Serologic diagnosis of LAC encephalitis has been accomplished using an indirect immunofluorescent assay (IFA) and an enzyme-linked immunosorbant assay (ELISA). The IFA test detected antibody in the acute phase serum specimen of 65 percent of the cases; HI detected antibody in 58 percent. IFA also detected antibody in 4 of 11 CSF samples. Studies are underway to determine the immunoglobulin class of the detected antibodies.